1. I've never heard of Whilhelmine Germany either...
2.
My first thought was the 'een' pronounciation, and i'd say that most people in English speaking countries would do the same.
3. I like it more than Wilhemina as a name, but I've always liked the nn
Mina, and you could really use that with
Wilhelmine... You've still got
Willa as an option though, which I also love.